1. Field of the Invention
This invention relates to power input controls for motors, and particularly to an improved control which varied input power to an AC induction motor responsive to a voltage-current phase (power factor) command signal, and wherein a phase comparison is made by combining the voltage and current derived square wave signals.
2. General Description of the Prior Art
In a previous patent application, now U.S. Pat. No. 4,052,648, the applicant described a power reduction system for less than fully loaded induction motors wherein the phase angle between current and voltage, motor power factor, was controlled. In the prior system, the voltage supplied to the motor and resulting current through the motor was sampled, and a pulse marking the trailing edge of each half cycle of current was developed and compared in phase with a square wave corresponding to the voltage waveform. This required significant processing of the current signal before phase comparison, and, of course, added to the cost of the system. Further, the immediate circuitry controlling the power switching device used, a triac, employed a rather complex oscillatory circuit for triggering the traic. This was determined desirable to assure that when once turned on, the traic remained on for the remainder of each half cycle of input voltage. Still further, in some applications, the response time of the prior system was too slow to respond to rapidly applied motor loads and to initial motor turn-on.
It is the object of this invention to provide a simpler system, that is, one requiring fewer components, and to provide one particularly adapted to respond to conditions wherein motor loads are abruptly incurred, and to increase the speed of full motor turn-on.